The sauropods were the more advanced members of the Sauropodomorpha and shared a common ancestor with the "prosauropods." The "prosauropods" maintained a basic body plan that grew to no more than about 35 feet (10.5 m) in the largest species. They had a jaw joint that was lower than the tooth line of the upper jaw, had three sacral vertebrae, and had longer hind limbs that permitted them to move about on two legs with relative ease. It was on these basal traits that sauropods evolved adaptations that eventually led to their ability to grow to enormous size and to process food to feed their presumably huge appetites.
Anatomical traits of the sauropods that made them diagnosti-cally different from the "prosauropods" and other dinosaurs include the following adaptations.
Quadrupedal posture with complementary changes to limb structure. American paleontologists Paul Sereno and Jeffrey Wilson published an extensive comparative analysis of the anatomical traits of sauropods in 1998. Noting that nearly all saurischian dinosaurs prior to sauropods—theropods and "prosauropods" alike—were largely bipedal animals, they focused their definition of the sauro-pods on those features that enabled them to function as full-time quadrupeds. The proportions, bone shapes, flexibility, and joint structures found in the legs of sauropods are significantly different from those of "prosauropods" and theropods. One telltale sign of a sauropod is that the femur (the upper hind limb bone) is straight and longer than the tibia (the lower hind limb bone); this was one of the many limb adaptations that enabled the bearing of great weight yet allowed the animals to move about with relative ease.
Four or more sacral vertebrae. Having three sacral vertebrae—the fused backbones connected to the pelvis that provided strength—as is the case in the "prosauropods," is considered an ancestral trait of the sauropodomorphs. Sauropods have four or more sacral vertebrae.
(continues on page 116)
THINK ABOUT IT
Two of the most famous paleontologists in post-Civil War America were Edward Drinker Cope of Philadelphia and Othniel Charles Marsh of New Haven, Connecticut. Both men studied dinosaurs found in the American West in the 1870s and 1880s. After several active years of exploring for fossils in person, both Cope and Marsh found even greater success by hiring crews of fossil hunters to find and dig the bones for them. Though friends as young men, the two became bitter rivals, and their dislike of each other lasted their whole adult lives. Cope and Marsh were fierce competitors in the discovery and naming of new dinosaurs.
In 1877, a schoolteacher named Arthur Lakes wrote to Marsh about some large fossil bones that he had found near the town of Morrison, Colorado. Marsh was curious, but instead of jumping at the chance, he politely offered to identify the bones if Lakes would send them to Connecticut. No deal was struck.
Before long, Lakes discovered more bones, including what looked like a gigantic leg. He wrote to Marsh again. Lakes estimated the total length of the animal at 60 to 70 feet, a figure that must have sounded unbelievably big at the time. This time, Lakes took more direct action. He packed up 10 crates of the bones and shipped them to Marsh at Yale in New Haven, thereby hoping to ignite the scientist's curiosity and loosen his wallet.
As a fallback, Lakes also sent a few of the spectacular bones to Cope in Philadelphia, and Cope was the first to act. Delighted at his good fortune, Cope was unaware that Marsh already had been offered the same fossils. Cope set to work to write a scientific description of the dinosaur, but the dinosaur was not his for long.
Marsh became aware of Lake's advances to Cope and immediately took action to gain the upper hand. Marsh sent Lakes a check for 100 dollars. He also warned Lakes that he wanted the entire specimen, including the bones that Lakes had sent to Cope. Unfortunately for Cope, he had not yet paid Lakes for any of the bones, so he had to pack them up and send them to Marsh in Connecticut.
So it was that Othniel C. Marsh identified his first new dinosaur without having set one foot out of New Haven, Connecticut. It was a long-necked sauropod, the first of many such giants to be discovered in the Late Jurassic-age deposits of the western United States. Marsh named
the animal Titanosaurus ("Titan lizard"); it later came to be called Atlanto-saurus. Marsh did not have evidence of the animal's long neck. Even so, he declared that this was the largest of "any land animal hitherto discovered." He thought that the complete dinosaur would measure about 50 to 60 feet long from its nose to the tip of its tail.
Cope's disappointment over the Lakes dinosaur did not last long. About 80 miles south of the Lakes location, near Canon City, Colorado, a school superintendent named O.W. Lucas discovered another fossil bed filled with extremely large dinosaur bones. These fossils dated from about the same time as those from Marsh's quarry, but they were in better condition and easier to dig out. Fortunately for Cope, Lucas contacted him first. Cope immediately bought the first sample bones sent to him by Lucas and hired a crew to help dig the bountiful site. By late summer 1877, Cope had turned the tables on Marsh. He announced the discovery of Camarasaurus, another giant, "which exceeds in proportions any other land animal hitherto described, including the one found by Professor Arthur Lakes." Cope also found the neck of Camarasaurus, making it one of the best known of the early sauropod discoveries in North America.
(continued from page 113)
This condition began to appear in some of the basal sauropods discussed below.
Enlarged claw and weight-bearing feet. The first digit—the inside "toe"—of each sauropod foot is fitted with an enlarged claw or ungual. There is a marked reduction in the size of the unguals from the inside to the outside toes. The feet and ankles are compact, yet sturdily built as weight-bearing structures.
Shift in the position of the nostrils to higher up on the skull. There was a gradual shift in the position of nostrils from the front of the snout, as in "prosauropods" and theropods, to a place higher up on the top of the skull. In later sauropods, such as the brachiosaurids and diplodocids, the nostrils had migrated well up on the skull and were placed either in front of or even between the eyes on top of the skull. The reason for this changing position in the nostrils is thought to be related to eating. With the nostrils out of the way of the oral cavity, it became easier for sauropods to eat uninterruptedly, without having to pause for a breath. This was a highly efficient adaptation for such huge animals that probably needed to maximize their feeding time each day.
Changes to the jaws and teeth. In "prosauropods," the rows of teeth joined at a fairly sharp angle at the front of the mouth. In sau-ropods, the front of the mouth was broader and more rounded. The sauropod jaw hinge, located higher than that in "prosauropods," allowed the teeth of the upper and lower jaws to rub together, providing a gripping bite. Although sauropods' teeth were also designed for plucking and not grinding plants, the flexibility of their jaws and the coordinated contact of their teeth improved their ability to consume vegetation. The teeth of sauropods eventually varied among taxa, from broad and spatulate in some specimens to more peglike in others. All sauropods had four premaxillary teeth.
Increased pneumaticity of the vertebral column. During the course of their evolution, the sauropods developed remarkable pneumaticity—the presence of concavities and spaces—in their backbones. This lessened their weight without sacrificing strength. This tendency was especially pronounced in the neck and back vertebrae. Adaptations that lead in the direction of improved pneumaticity of the vertebral column were evident in some of the basal sauropods.
Elongation of the neck. The gradual elongation of the neck is one of the hallmarks of sauropod evolution. With longer necks, these giants increased the reach of their heads to the sides in some species and to greater heights in others, presumably to improve the efficiency of gathering food.
Many of the above traits of the most familiar sauropods began to appear in their earliest ancestors. A gradual increase in the body size was clearly under way by the Early Jurassic. Even the smallest of the early sauropods, such as Blikanasaurus and Vulcanodon, were about the same length as the mid-sized "prosauropods" at 20 to 22 feet (6 to 6.5 m). The Early Jurassic sauropod Gongxianosaurus hinted at the future sauropod tendency toward gigantism by near-ing a length of 50 feet (15 m). As quadrupeds, the early sauropods were probably somewhat stockier than most "prosauropods." The bulkier body design and eating tools of the sauropods may have given them a competitive edge when food resources were scarce, such as in times of drought, especially if both kinds of dinosaurs were eating plants of the same variety and of similar heights, which was probably the case. If such a plant-eating contest took place during the Early Jurassic, sauropods must have been the heavyweights, utilizing their effective jaw design to strip leaves from plants and fill their bulky bodies with great ease.
Was this article helpful?